Method for gantry angle repositioning on rotational teletherapy equipment

Repositioning errors for given angles have been found to be as large as +/-0.5 degrees. Errors of this magnitude are undesirable and unnecessary. To overcome this difficulty, a small laser was mounted to the counterweight of our accelerator with its light beam projected at 90 degrees to the axis of the x-ray beam. The light beam is used as a lever arm to indicate large distances (0.125 in.) for small changes in gantry angle (0.1 degrees). Gantry angles can easily be respositioned within 0.1 degrees. Routine use of the laser beam indicator has reduced the time taken to reposition the gantry.